Method of pulverizing plastic materials



Feb. 1, 1966 .1. PASTEKA 3,232,543

METHOD OF PULVERIZING PLASTIC MATERIALS Filed July 19, 1963 3 Sheets-Sheet l Fig.7

INVENTOR Jose? PA-STEKA.

Feb. 1, 1966 .1. PASTEKA 3,232,543

METHOD OF PULVERIZING PLASTIC MATERIALS Filed July 19, 1963 3 Sheets-Sheet 2 27 2s Fi .7 F #11 g 1s ,;-2s

I LLZZL m INVENTOR Jose F P619 Feb. 1, 1966 .1. PASTEKA 3,232,543

METHOD OF PULVERIZING PLASTIC MATERIALS Filed July 19, 1963 5 Sheets-Sheet 3 Fig.8

0 0 29 Fig.9

INVENTOR JoseF WSTEWA Mar United States Patent Oflice 3,232,543 Patented Feb. 1, 1956 3,232,543 METHUD F PULVERHZING PLASTIC MATEREALS .losef Pasteka, Am Laubersherg 26, Steinheim am Main, Germany Filed July 19, 1963, Ser. No. 296,181 laims priority, application Germany, July 21, 1962,

C 27,523 2 Claims. ((11. 241-46) The present invention relates to a method of pulverizing plastics as well as synthetic and natural rubbers.

The aforementioned materials are characterized by great toughness at normal and elevated temperatures and are extremely difiicult to grind. Thus, liquid nitrogen has been used heretofore to maintain the grinding stock in a brittle state.

The cost of the ground material is considerably increased by that of the liquid nitrogen.

Accordingly, a primary object of this invention is to provide a method which permits economical pulverizetion of materials of the aforementioned type.

The method of the invention includes the steps of rough-crushing the stock to be pulverized to a grain size of approximately 3 to 6 mm; introducing the pre-crushed material into an air stream in such a proportion that the grinding process carried out thereafter takes place at a temperature equal to approximately one half the softening temperature of the grinding stock in degrees centirade; and removing the material from the grinding zone as soon as it has been ground to a particle size between 50 and 500 microns, together with at least a portion of the air stream.

Thus the present method provides for pulverization of the material in a state in which it is relatively far from becoming soft thereby avoiding any excessive energy consumption by the grinding stock itself.

Another advantage of the novel method is that the ground stock is removed from the mill immediately after it has been reduced to a grain size of 50 to 500 microns. Thus an accumulation of pulverized stock in the mill, which would cause the temperature to rise, is avoided.

According to one feature of the invention, the pulverizer substantially consists of a housing having a central feeding aperture and of a rotor rotatably mounted inside the housing. The rotor is circumferentially enclosed by a screen. The rotor is provided with blade members which move relative to the screen whose opening size corresponds to the desired fineness of the ground stock.

Pulverization of the material is effected by a combined impact-and-cutting action between the cutting edges of the rotor blades and inner edges of ridge or saw-tooth sections of the screen.

Further details and advantages of the invention will become apparent from the following specification, when read in coniunction with the appended drawings wherein FIG. 1 shows the rotor of a mill according to the invention in axial section;

FIG. 2 shows a modified rotor in a fragmentary view analogous to that of FIG. 1;

FIG. 3 is a fragmentary front-elevation of the rotor of FIG. 1 and of associated other elements of the mill;

FIG. 4 is a perspective view of the rotor;

FIG. 5 is an enlarged detail view of the rotor and of the associated screen;

FIG. 6 is a perspective view of a part of the screen;

FIG. 7 shows a mill in elevational section on its axis, and its controls in a conventional manner;

FIG. 8 is a circuit diagram of modified controls; and

FIG. 9 is another circuit diagram of controls for the mill of FIG. 7.

Referring now to the drawing in detail, and initially to FIG. 7, there is seen a pulverizing mill of the invention which has a stationary housing 18. A bearing 20 mounted on the housing supports a shaft 19 one end of which carries a drive pulley 21 outside the housing whereas the other end of the shaft carries a rotor within the housing.

The rotor, seen in more detail in FIGS. 1 and 4, includes a disk 1 fastened to the inner end of the shaft 19 and peripherally connected to a coaxial ring 3 by rods 2, omitted from the showing of FIG. 7 for the sake of clarity. Axially aligned radial slots 4 in the ring 3 and the disk 1 receive respective end portions 6, 6 of circumferentially spaced blades 5 which are radially offset from the exposed edges of the blades. The blades are retained in the slots 4 by annular covers 7, and their end portions 6, 6' rest on corresponding faces of the ring 3 and the disk 1.

In the modified rotor partly shown in FIG. 2, a supporting ring 4 is interposed in the slots 4 between the end portions 6" of each blade and the supporting surfaces of the ring 3 and the disk 1, the blade being of symmetrical shape.

FIG. 3 shows examples of blade configurations in the mill of the invention. The exposed edges of the blades 3 are of rectangular cross section. The blade 9 has an exposed edge formed by a leading face that extends in an axial plane and a trailing face which obliquely intersects the leading face. In the blade 10, the trailing face extends in an axial plane, and the leading face is obliquely inclined.

Reverting now to FIG. 7, the rotor is coaxially enveloped in the housing 18 by a stationary screen 11. The screen is a corrugated sheet member, as best seen in FIG. 6, whose ridges 13 are axially elongated. Rows of openings 12 are formed in the screen portions which face in the direction of rotor rotation. The axes 14 of the openings are approximately tangential to the rotor, as best seen in FIG. 5.

The radial wall 22 of the housing 18 opposite the shaft 19 has a central aperture fitted with an inlet duct 15. Vent openings 23 are arranged in the wall 22 about the duct 15 and are provided with a common annular closure plate 24 which may be rotated by a motor 25 to open and close the orifices of the openings 23.

A bin for material to be ground 'is arranged above the mill and is provided with an electrically operated chute 17 leading to the duct 35. The open bottom of the housing 18 forms a discharge duct 26.

A temperature sensing device 16, whose sensing element is a thermistor, is installed on the screen 11 at the discharge duct 26. It controls the operation of the chute 17 and of the motor 25 to increase the air supply to the grinding zone and to decrease the rate of supply of the material to be ground when the temperature in the grinding zone, as sensed by the device 16, exceeds a desired limit, and to decrease the air supply and increase the solids supply at lower temperatures.

The sensing device 16 is connected to a transductor 27 by leads 2'7. Leads 25' connect the transductor to the motor 25, and lead 17' to the chute 17. A resistor 28 in one of the leads 17 modifies the response of the chute 17 to the temperature sensed by the device 16.

In the modified control circuit shown in FIG. 8, the sensing device 16 and transducer 27 control only the motor 25 Whose response may be varied by adjusting a variable resistor 29 in one of the leads 25'. In the circuit of FIG. 9, the motor 25 as well as the chute 17 are controlled by the sensing device 16, and the response of either may be adjusted by respective resistors 29, 28.

The following examples illustrate the operation of the mill illustrated in the drawing.

3 EXAMPLE 1 Polyethylene granules having a particle size of approximately 4 mm. were fed to the inlet duct 15 while the mill was rotating at 3100 rpm. The rotor had a diameter of 680 mm., and the screen openings 12 were 0.8 mm. The throughput of polyethylene was about 125 kg. per hour, the power consumption 70 H.P. With an ambient temperature of 14 C., the temperature sensed by the thermistor 16 was held at 43 C. The air flow through the mill was maintained at approximately 70 m. per minute. The particle size distribution of the product obtained was determined by sieve analysis.

Sieve analysis Oversize products Microns: in percent 600 1.5

Under 200 19.0

EXAMPLE 2 The mill described in Example 1 was fitted with a screen having apertures of 0.5 mm., and was fed rigid polyvinylchloride in granules of mm. size while rotatihg at 3100 r.p.m. When the drive motor of the mill consumed 90 arnp'eres, the mill produced 115 kg. of powder per hour, at a current consumption of 110 amperes, the throughput was 145 kg. per hour. The corresponding power consumption was 60 and 80 H.P. respectively. An operating temperature of 49 C. to 61 C. was maintained with an ambient temperature of 24 C.

Sieve analysis Oversize products Microns: in percent 400 Traces 300 2.0

200 23.0 100 62.0 Under 100 13.0

while the novel features of the invention have been described as applied to preferred embodiments, it will be obvious that modifications of the device illustrated may be made without departing from the spirit and the scope of the invention.

What is claimed is:

1. A process for pulverizing thermoplastic organic polymer material which comprises:

(a) precrushing said material to a particle size of approximately 3 to 6 millimeters;

(b) admixing said material to a stream of gas having a temperature substantially lower than one half of the softening temperature of said material in degrees centigrade in a controlled proportion;

(c) feeding the stream of gas having said material admixed thereto to a grinding zone;

(d) grinding said material in said zone to a particle size between and 500 microns;

(e) sensing the temperature of the material being ground;

(if) controlling said ratio in response to the sensed temperature to maintain said sensed temperature substantially below said softening temperature; and

(g) removing the ground material from said grinding zone as soon as the particle size thereof is be tween 50 and 500 microns.

2. A process as set forth in claim 1, wherein said sensed temperature is maintained at approximately one-half of said softening temperature.

References Cited by the Examiner UNITED STATES PATENTS 1,945,054 1/1934 MacGregor et a1. 241-88 2,427,903 9/1947 Crites 24133 2,602,594 7/1952 Hesse 241- X 2,726,045 12/1955 Hinerfeld 24116 2,766,939 10/1956 Weston 241-19 2,844,327 7/1958 Baternan et al 241-65 X 2,916,215 12/1959 Weston et al. 24119 2,974,883 3/1961 Engel 24119 X 2,977,055 3/1961 FaWCett 24186 3,067,957 12/1962 Erck et al. 241-23 3,088,683 5/1963 Joseph et a1 24186 3,090,568 5/1963 Wetmore 24188 X 3,123,312 3/1964 Palyi 24186 X LESTER M. SWINGLE, Primary Examiner.

WILLIAM W. DYER, IR., ROBERT C. RIORDON,

Examiners.

D. L. MAXSON, HARRY F. PEPPER, 111.,

Assistant Examiners. 

1. A PROCESS FOR PULVERIZING THERMOPLASTIC ORGANIC POLYMER MATERIAL WHICH COMPRISES: (A) PRECRUSHING SAID MATERIAL TO A PARTICLE SIZE OF APPROXIMATELY 3 TO 6 MILLIMETERS; (B) ADMIXING SAID MATERIAL TO A STREAM OF GAS HAVING A TEMPERATURE SUBSTANTIALLY LOWER THAN ONE HALF OF THE SOFTENING TEMPERATURE OF SAID MATERIAL ADCENTIGRADE IN A CONTROLLED PROPORTION; (C) FEEDING THE STREAM OF GAS HAVING SAID MATERIAL ADMIXED THERETO TO A GRINDING ZONE; (D) GRINDING SAID MATERIAL IN SAID ZONE TO A PARTICLE SIZE BETWEEN 50 AND 500 MICRONS; (E) SENSING THE TEMPERATURE OF THE MATERIAL BEING GROUND; (F) CONTROLLING SAID RATIO IN RESPONSE TO THE SENSED TEMPERATURE TO MAINTAIN SAID SENSED TEMPERATURE SUBSTANTIALLY BELOW SAID SOFTENING TEMPERATURE; AND (G) REMOVING THE GROUP MATERIAL FROM SAID GRINDING ZONE AS SOON AS THE PARTICLE SIZE THEREOF IS BETWEEN 50 AND 500 MICRONS. 